Hi Blokey,
I know you are being bombarded with advice here, but I personally would go down there, preferably with your solicitor, and insist on a receipt for the money you have paid. Don't leave without it. Call him up, tell him you are coming round the next day to pick it up, and then sit there until he gives you the receipt. That is surely the first step. Threatening to record phone calls and removing the boat in 7 days etc, will sound like empty threats if you can't go through with it. Its a horrible situation to be in, and chances are the guy is just an unreliable loser and you'll get your boat in the end. Hang on in there.